Early on in the procedural history of total hip arthroplasty, all polyethylene acetabular replacement components were designed without metal backing and were routinely stabilized with cement. The outer cemented surface of the component was provided with shallow, cement-adhering irregularities of some sort, usually in the form of narrow and shallow circumferentially oriented channel indentations. The spherical prosthetic, head-containing dimension of the socket part of the implant was commonly limited to less than a hemisphere in the interest of avoiding premature, motion-induced abutment of the prosthetic femoral neck against an otherwise overly extended marginal outlet of the implant which would obstruct the functional range of rotational joint motion and induce the associated propensity for anterior or posterior dislocation of the replaced joint.
The not infrequent occurrence of latent procedural failure owing to implant loosening associated with degradation and separation of the cemented bone-prosthetic interface led to the introduction and subsequent use of metal-backed polyethylene acetabular components in the early 1980s for the intended purpose of ameliorating the incidence of this complication. The metal-backed part of the composite component was made available with either a porecoated acetabular interfacing surface that depended on latent bone growth to stabilize the prosthesis with the patient's acetabulum or without a porous coated surface that required cement fixation to provide implant stability.
In the meantime, the reduction in the spherical marginal extension of the implant to less than a hemisphere is, of course, accompanied by a commensurate reduction in the overall depth of the prosthetic head-constraining socket, and, consequently, the replaced hips remained vulnerable to the propensity for postoperative dislocation. Early in the 1970s, an attempt was made to counter this complication by extending the spherical continuation of the socket margin a bit beyond a hemisphere, i.e., sufficient enough to provide a snap-fitted containment with the prosthetic femoral head. In this initial concept of a snap-fitted articulation, the marginal extension was distributed evenly around the total circumferential margin of the socket outlet. As a consequence of problems that were associated with an articular constraining arrangement of this sort, the concept of a snap-fitted articulation was then abandoned after a short period of use. However, the continuing problem of postoperative joint dislocation has been addressed more recently by other innovators who have designed the acetabular component with a superiorly positioned, non-snap-fitted marginal extension for the purpose of merely providing a structural block against dislocation. Nevertheless, in the art of metal-backed acetabular cups, a polyethylene insert in the multi-component, metal-backed acetabular cup was developed with a snap-fitting arrangement with cut-out rims. See, BioPro, Inc., "PSL Physiological Stress Loading Total Hip Replacement System Utilizing the Horizontal Platform Supported Concept and the Cox Comb Acetabular Component," 1991; BioPro, Inc., "PSL Physiological Stress Loading Total Hip Replacement System Utilizing the Horizontal Platform Supported Concept," 1997. That insert, which has marginal cut-outs and was developed to eventually include a snap-fitting hood arrangement, contains, however, a smooth back save one round, centrally located button protruding therefrom, and a dovetail slot in an elevated rim portion, which smooth back rests against the smooth inside of the metal backing and which button projects through a hole in the metal backing and which dovetail slot is snap locked into a corresponding male post on the metal backing to assist in fastening the insert component to the metal backing component. Note, both BioPro, Inc. references, at pages 4 (FIGS. H, I); and the 1997 BioPro, Inc. reference, at page 7 (Coxcomb Acetabular Cups, illustration).
Drawbacks to the heretofore listed, variable methods of designing and fixation of an acetabular replacement component are multifactorial and are generally enumerated as follows:
First, the nature and distribution of the shallow irregularities on the cemented outer surface of the polyethylene component makes it technically difficult, if not impossible, to assure a maximally efficient implant-stabilizing, bone-to-prosthesis cement mantle, i.e., a cement mantle that is symmetrically distributed to provide a consistently even and adequate thickness of cement throughout the total dimension of the bone-prosthesis interfacing interval.
Second, acetabular components which are designed with a femoral head-constraining marginal outlet that is reduced to less than a hemisphere are prone to postoperative dislocation. Although a relatively infrequent occurrence, postoperative dislocation of a replaced hip is an inordinately serious and disheartening complication. Aside from the severe discomfiting and emotional effect of the ordeal on the patient and surgeon alike, this complication has a substantial adverse impact due to the not insignificant costs associated with the required added surgical intervention and hospital care.
Third, polyethylene acetabular components that were designed with a complete circumferentially extended snap-in marginal outlet produced yet an earlier premature stage of marginal abutment against the neck of the femoral component, and an associated, commensurate reduction in the range of rotational motions, which in turn induce the propensity for a "leveraged-out" anterior or posterior joint dislocation, notwithstanding the snap-fitted arrangement. Furthermore, repetitive, motion-induced abutment between the femoral neck and the extended acetabular margin, particularly with rotary joint motions, resulted in erosive degradation of the repetitively traumatized, overextended margins of the acetabular cup outlet, consequentially producing yet another source for the proliferation and the associated detrimental effects of foreign body debris.
Fourth, cemented metal-backed acetabular components have had no demonstrable salutary effect on the incidence of implant loosening complications.
Fifth, long-term experience with the use of uncemented porecoated metal-backed components, although shown to be effective in stabilizing the implant, has demonstrated a substantive acceleration in the rate of polyethylene wear, and an associated significant increase in the incidence and severity of debris-induced periprosthetic osteolysis, occasionally with catastrophic consequences to the patient.
Sixth, the concept of a superiorly oriented, non-snap-fitted marginal extension of the implant, although reasonably effective in reducing the incidence of dislocation if properly aligned, is highly technique-dependent, i.e., relative to the appropriate rotational positioning of the extended head-restraining implant rim. This is specifically relevant to the inability to predetermine with any absolute degree of certainty as to whether a given hip is, or will be, vulnerable to an anterior versus a posterior joint dislocation. For instance, an inadvertent, erroneously suspected propensity for anterior dislocation that is managed by a precautionary antero-superiorly positioned rim extension may result in a marginally unguarded posterior dislocation, or vice versa.
It would be desirable to overcome such drawbacks and provide an improved socket implant, the lack and need for which is of long standing in the art. It would be especially desirable to be able to apply improvements to the human hip.